For high-throughput and low-cost production, the thickness of solar wafers consisting of photovoltaic elements has been reduced to 200 μm or below. Thinner substrates give rise to lower wafer stability and a higher risk of wafer breakage. An effective method to detect cracks and even micro-cracks is therefore important in order to reject those wafers that have a higher potential to fail.
Some approaches to detecting the presence of micro-cracks which rely on non-visual inspection include the use of ultrasonic energy, thermal energy or heating, thermosonic energy and mechanical flexing. Furthermore, there are visual approaches such as Electroluminescence (EL) inspection and Photoluminescence (PL) inspection, which are two popular visual methods. Electroluminescence inspection requires electrical contact and then can only be used in a finished solar cell. Photoluminescence requires a uniform illumination source with appropriate non-uniformity correction, and the shot-to-shot reproducibility is usually poor.
US Patent Publication number 2005/0252545 A1 entitled “Infrared Detection of Solar Cell Defects under Forward Bias” discloses a cell inspection system which applies a forward-bias current to cause heating. The resulting thermal image of applies a forward-bias current to cause heating. The resulting thermal image of each cell is then analyzed with an infrared camera to inspect the cell for cracks. However, this approach is relatively expensive and slow.
Apart from the above approaches, a purely visual approach has also been adopted in the prior art, although it is generally less accurate. Since silicon is semitransparent in the near-infrared (NIR) spectrum, NIR backlight inspection is widely used for incoming raw solar wafer but suffers from various disadvantages. For instance, a polycrystalline material on the substrate typically displays intrinsic heterogeneous features, and some features have patterns that are similar to a micro-crack pattern. Hence, it is difficult to distinguish a micro-crack from the general surface texture. Moreover, the camera resolution needs to be very high in order to distinguish a micro-crack. Even if a camera that is capable of achieving up to 8,000×8,000 pixels-per-substrate is used, the pixel resolution is still only 20 μm. This is far below the resolution required for detecting micro-cracks.
To illustrate the above shortcoming, FIG. 1 is a side view of a conventional inspection system 100 using near-infrared (“NIR”) backlighting 106 to detect cracks on a substrate 102. Homogeneous NIR backlighting 106 from a single light source is projected from underneath a substrate 102 such as a photovoltaic element. The light from the NIR backlighting 106 penetrates the substrate 102 where a micro-crack 104 is present, towards an imaging optical device 108. The imaging optical device 108 then transmits the light to an image grabbing sensor 110.
FIG. 2 is a photograph showing an image of a substrate 102 that has been illuminated with NIR backlighting 106 using the conventional inspection system 100. The substrate 102 has multiple heterogeneous features due to its polycrystalline grain textures. Some of these features have patterns that are similar to a micro-crack pattern, and it is difficult for the conventional inspection system 100 to discern the presence of a micro-crack 102. It would be advantageous to develop a method and system for identifying micro-cracks on substrates having such intrinsic heterogeneous features more effectively and accurately.